140 PROCEEDINGS OF THE BOSTON MEETING 



GEOLOGY OF THE KATMAI REGION, ALASKA, AND THE GREAT ERUPTION 



OF 1912 



BY CLARENCE N. FENNER 



(Ahstract) 



During the summer of 1919 a party of three from the Geophysical Labora- 

 tory cooperated with the expedition sent by the National Geographic Society 

 to the Katmai region, and made geological and chemical studies of the phe- 

 nomena connected with the great eruption of a few years ago and of the 

 fumarolic activity still going on. These studies have not yet been fully worked 

 up and only a preliminary report can be made, but an account was given of 

 some of the more interesting geological features. This includes the general 

 geology of the district and the arrangement of the volcanic chain ; the nature 

 of the lava-masses which have built up the volcanoes as contrasted with the 

 material blown out recently ; the so-called mud-flow in the Valley of Ten 

 Thousand Smokes and its probable origin ; fumarolic activity ; the extruded 

 plug of Novarupta ; the present form of the crater of Katmai and its relation 

 to the great eruption ; the stratified ash which covers the region. 



Eead from manuscript, with lantern slide illustration. 



Discussed by Messrs. J. E. Spurr, Lawrence Martin, and A. H. Brooks. 



ANORTHOSITE-GABBRO IN NORTHERN NEW YORK 

 BY WILLIAM J. MILLER 



(Ahstract) 



A body of Precambrian anorthosite-gabbro ly^ miles long, in Saint Law- 

 rence County, New York, exhibits a number of features of special interest. 

 Much of the rock is a normal dark medium-grained to coarse-grained massive 

 gabbro, but it commonly varies to anorthosite-gabbro, and even to nearly 

 white anorthosite, with only 5 to 1 per cent of dark minerals. From facies 

 with no foliation there are all gradations into those which are very highly 

 foliated. The variants in composition and structure are mostly in zones or 

 bands more or less sharply separated from each other, x^ll of the principal 

 variations in composition, structure, and texture may be observed within a 

 distance of a few rods. 



One zone of the anorthosite-gabbro, a rod wide and about 200 feet long, is 

 highly fractured by curving cracks, which are filled in part by dikes of nearly 

 pure plagioclase anorthosite and in part by dikes of hornblendite. These filled 

 cracks show a remarkable festooned arrangement. 



Dikes of many kinds, with and without sharp contacts, cut the anorthosite- 

 gabbro. These include white dikes consisting chiefly of plagioclase and scapo- 

 lite, hornblendite, hornblende-rich gabbro, pegmatite (acidic to basic), and 

 granite. 



The gabbro is surrounded mostly by a younger granite, and along the bor- 

 ders between the two there are some fine displays of injection gneisses. Dikes 

 of pure pyroxenite also cut this bordering granite. 



